With the modern heat treatment processes for material in dust-like form, the material is generally calcined or burnt in rotary kilns. In front of the kilns are connected heat exchangers for the recovery of the heat, which is contained in the waste gases of the kilns.
For endothermal processes as for example the manufacture of cement, various heat exchangers have been proposed for this purpose, as for example, cyclone preheaters, and shaft preheaters. Some of these use the counter-current method, whereas others use the unidirectional flow method.
With these calcining plants, the heat is produced at a location where it is not actually required, -- in the sintering zone of the rotary kiln. The transporting of the heat to the position where it is to be used, -- to the deacidifying zone, and further for preheating purposes, is carried out in the manner which so far has not been technically satisfactory, in particular to conducting all the combustion gases through the rotary kiln. In this case, the gases are contained in these waste gases and which are driven out of the untreated material, such as sulphur dioxide and carbon dioxide, as well as the vaporizable, mineral constituents, such as alkali oxides, chlorides and sulphates. The gases are also entrained and lead to the colder section of the kiln or in the lower part of the heat exchanger to formations of deposits which sensibly disrupt the calcining operation in practically all works. In addition, the sulphur quantities which are introduced with the raw material and the fuel are combined in the kiln with lime. As a result, the sulphates thus forming cannot be removed with the waste gases and, by circulations in the kiln, lead to the lowering of the melting point of the clinker. It has been found that such sulphur cycles are able to lower the melting point of the raw material by some hundreds of degrees, so that often there are considerable interruptions in the calcining operation because of these low-melting compounds, due to ring formations and excessive deposits.
In addition, it has been found in connection with these systems that infiltrated air occurs at the inlet and outlet of the rotary kiln. This is caused by the difficulty in sealing off the rotating parts from the fixed parts, through the method of procedure. The infiltrated air increases the fuel requirements of the system, firstly because it replaces the hot air which can be utilized from the condenser by cold air, and secondly because it has to be heated to process temperature. This leaves the system at higher temperature, and as a consequence causes an additional loss of heat.
The efficiency of such kilns is determined by the quantity of fuel which can be burnt per unit of time in the kiln. The fuel quantity depends on the air quantity which can be sent through per unit of time, and also on the kiln volume charging M, expressed, for example, in tons of clinker per 24 hours. The two values, the fuel quantity and the air quantity, produce the fuel heat flow. According to the prior art, the highest possible fuel heat flow amounts to Q.sub.Br = 0.75 .sup.. 10.sup.6 D.sub.i.sup.3 (kcal/h) (D.sub.i = internal diameter of the kiln), see R. Frankenberger: "Ofenabmessungen, Durchsatz and Waerme-bedarf von Schwebegas-Warmetauscheroefen", Zement-Kalk-Gips 1967, pages 453-458. According to this same work, the maximum kiln output with the present-day operating methods is M = 1.7554 .sup.. V.sub.i (V.sub.i = internal volume of the kiln). The burning or calcining efficiency is inter alia limited by the fact that, with a constantly increasing air capacity, the speed of the gases in the kiln finally becomes so high that the powdered material in the kiln is also entrained. As a consequence it is no longer possible to maintain any regulated calcining operation. The fuel heat flow is, therefore, determined in practice by the highest permitted gas velocity in the kiln.
For these reasons, always larger kilns have to be constructed for increasing outputs. This in turn means higher driving costs, and also more difficult lining operations, as well as an increased tendency to disruptions in the burning operation caused by ring formations.
In the aforementioned work by Frankenberger, it is shown in FIG. 2 how the kiln volume has to be increased when the capacity is raised.
With one known process for the manufacture of cement clinkers, the endothermal process of the deacidification is partly carried out in a fluidizing furnace separated from the rotary kiln (German Patent No. 1,251,688). In this arrangement, the fuel is supplied to the fluidizing furnace, for example, in the form of oil shale. The quantity of fuel to be introduced in this case is, however, limited by the fact that the air for combustion, which is to be conveyed through the rotary kiln, must not exceed a certain velocity, at which the powder is also entrained. It is, of course, possible for the rotary kiln to be chosen so as to be considerably shorter for the same capacity. However, because of the limitation of the gas velocity, the diameter of the rotary kiln must increase with increasing capacity, so as to keep the gas velocity below a certain limiting value.
Accordingly, it is an object of the present invention to improve the loading, i.e. the specific capacity of a rotary kiln installation, in tons per cubic meter of kiln volume and permit of time.